1. Field of the Invention
The present invention relates generally to a wireless mobile or system, and more particularly, to a method of transmitting and receiving channel state information wherein a user equipment measures radio channel quality and reports the measurement result to a base station in a wireless mobile communication system employing a multi-carrier multiple access scheme such as Orthogonal Frequency Division Multiple Access (OFDMA), and to a system in which a base station transmits to, and receives from, user equipments using multiple antennas.
2. Description of the Related Art
To meet the demand for wireless data traffic which has increased since the deployment of 4G communication systems, efforts have been made to develop an improved 5G or pre-5G communication system. Therefore, a 5G or pre-5G communication system is also called a “Beyond 4G Network” or a “Post LTE System.” A 5G communication system is considered to be implemented in higher frequency (e.g. mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, beamforming, massive Multiple-Input Multiple-Output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, analog beam forming, and are scale antenna techniques are discussed in 5G communication systems. In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, Device-to-Device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like. In a 5G system, Hybrid Frequency Shift Keying (FSK) and Feher's Quadrature Amplitude Modulation (FQAM) and Sliding Window Superposition Coding (SWSC) as an Advanced Coding Modulation (ACM), and Filter Bank Multi Carrier (FBMC), Non-Orthogonal Multiple Access (NOMA), and Sparse Code Multiple Access (SCMA) as an advanced access technology have been developed.
The Internet, which is a human centered connectivity network where humans generate and consume information, is now evolving to the Internet of Things (IoT), where distributed entities, such as things, exchange and process information without human intervention. The Internet of Everything (IoE), which is a combination of IoT technology and Big Data processing technology through connection with a cloud server, has emerged. As technology elements, such as “sensing technology,” “wired/wireless communication and network infrastructure,” “service interface technology,” and “security technology” have been demanded for IoT implementation, a sensor network, Machine-to-Machine (M2M) communication, Machine Type Communication (MTC), and so forth have been recently researched, Such an IoT environment may provide intelligent Internet technology services that create new value to human life by collecting and analyzing data generated among connected things. IoT may be applied to a variety of fields including smart home, smart building, smart city, smart car or connected cars, smart grid, health care, smart appliances and advanced medical services through convergence and combination between existing Information Technology (IT) and various industrial applications.
In line with this, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as a sensor network, Machine Type Communication (MTC), and Machine-to-Machine (M2M) communication may be implemented by beamforming, MIMO, and array antennas. Application of a cloud Radio Access Network (RAN) as the above-described Big Data processing technology may also be considered to be as an example of convergence between 5G technology and IoT technology.
In contrast to early mobile communication systems providing voice-oriented services only, advanced mobile communication systems may provide high-quality data and multimedia services based on high-speed packet data communication. To this end, several standardization organizations including 3rd Generation Partnership Project (3GPP), 3GPP2 and the Institute of Electrical and Electronics Engineers (IEEE) have been working to standardize enhanced 3rd Generation (3G) mobile communication systems. In recent years, various mobile communication standards including 3GPP Long Term Evolution (LTE), 3GPP2 Ultra Mobile Broadband (UMB) and IEEE 802.16m have been developed to support high-speed and high-quality wireless packet data services based pan multi-carrier multiple access schemes.
Existing enhanced 3G mobile communication systems such as LIE, UMB, 802.16m are based on multi-carrier multiple access schemes and utilize various techniques for increasing transmission efficiency, such as Multiple Input Multiple Output (MIMO), beamforming, Adaptive Modulation and Coding (AMC), and channel sensitive scheduling. These techniques may enhance transmission efficiency and increase system throughput by concentrating transmit power or adjusting the amount of data to be sent through multiple antennas according to channel quality, or by transmitting data to users with acceptable channel quality in a selective manner.
In most cases, such techniques work on the basis of channel state information between a base station (e.g. evolved Node B (eNB)) and a user terminal (User Equipment (UE), or Mobile Station (MS)), Hence, an eNB or a UE must measure states of the channel between the eNB and the UE. A Channel State Information Reference Signal (CSI-RS) is used for this purpose. An eNB is an apparatus located at a specific site for downlink transmission and uplink reception, and may perform transmission and reception for multiple cells. In one mobile communication system, multiple eNBs are distributed at geographically separated sites and each eNB performs transmission and reception for two or more cells.
To increase data rates and system throughput, existing 3G and 4G mobile communication systems such as LTE/LTE Advanced (LTE-A) may utilize MIMO technologies based on multiple transmit and receive antennas. In MIMO, multiple spatially separated information streams may be sent by use of multiple transmit and receive antennas. Transmission of multiple spatially separated information streams is referred to as spatial multiplexing. The number of information streams that can be sent through spatial multiplexing varies according to the number of antennas at the transmitter and the receiver, in general, the number of information streams that can be sent through spatial multiplexing is referred to as the transmission rank. In LTE/LTE-A up to Release 11, MIMO spatial multiplexing with 8 transmit antennas and 8 receive antennas may support up to rank 8 transmission.
FIG. 1 illustrates a communication system to which the present invention is applied.
In FIG. 1, a base station transmitter 100 may utilize several dozen or more transmit antennas to send radio signals. As shown in FIG. 1, transmit antennas are uniformly placed with a fixed spacing. The fixed spacing may correspond to multiples of half the wavelength of a radio signal being sent. In general, when transmit antennas are separated by a distance corresponding to one half the wavelength of the radio signal, signals sent by the transmit antennas receive influence from low correlated radio channels. The correlation between signals becomes lower with increasing distance between the transmit antennas.
In FIG. 1, several dozen or more transmit antennas installed in the base station transmitter 100 are used to transmit signals 120 to one or more UEs. Suitable precoding is applied to the transmit antennas so that signals are simultaneously transmitted to multiple UEs. In this case, a UE may receive one or more information streams. In general, the number of information streams that one UE can receive is determined according to the number of receive antennas of the UE and channel conditions.
For effective MIMO implementation, it is required for UEs to accurately measure the channel condition and interference and effectively send corresponding channel state information to the eNB. Upon reception of the channel state information, the eNB may determine the UEs to receive downlink transmission, data rates to be used, and precoding modes to be applied on the basis of the channel state information. When the schemes for channel state information transmission and reception used in the existing LTE/LIE-A system are applied to Full-Dimension MIMO (FD-MIMO) involving a large number of transmit antennas, an uplink overhead problem, which requires transmission of a large amount of control information in the uplink, may arise.
Time, frequency and power resources are limited in a mobile communication system. As such, allocation of more resources to reference signals may cause reduction of resources allocable to data traffic channels. This may reduce the amount of data being actually transmitted. In other words, enhancement of channel measurement and estimation may cause reduction of the amount of data being actually transmitted, degrading overall system throughput.
Accordingly, it is necessary to maintain a balance between resource allocation for reference signals and resource allocation for traffic channels so as to produce optimum performance in terms of overall system throughput.
FIG. 2 illustrates a radio resource with one subframe and one resource block serving as a minimum unit for downlink scheduling in the LTE/LTE-A system.
As shown in FIG. 2, the radio resource is composed of one subframe in the time domain and one Resource Block (RB) in the frequency domain. The radio resource includes 12 subcarriers in the frequency domain and 14 Orthogonal Frequency Division Multiplexing (OFDM) symbols in the time domain, and hence is composed of 168 unique frequency-time positions in total. In LTE/LTE-A, each frequency-time position in FIG. 2 is referred to as a Resource Element (RE).
The radio resource shown in FIG. 2 may be used to transmit different types of signals as follows.
A Cell-specific Reference Signal (CRS) is a reference signal that is periodically transmitted for all UEs within a cell and may be commonly used by multiple UEs.
A Demodulation Reference Signal (DMRS) is a reference signal transmitted to a specific UE. The DMRS is sent only when data is sent to the corresponding UE. The DMRS may include 8 DMRS ports in total. In LTE/LTE-A, ports 7 to 14 correspond to DMRS ports, and orthogonality between these ports is preserved through Code Division Multiplexing (CDM) or Frequency Division Multiplexing (FDM) so as not to cause interference with each other.
A Physical Downlink Shared Channel (PDSCH) is a downlink data channel used by an eNB to transmit traffic data to a UE and is mapped to REs not used for reference signal transmission in the data region of FIG. 2.
A Channel Status Information Reference Signal (CSI-RS) is a reference signal transmitted to UEs within a cell and used for channel state measurement. Multiple CSI-RS may be sent within a cell.
Other control channels (e.g., Physical Hybrid-Automatic Repeat reQuest (ARQ) Indicator Channel (PHICH), Physical Control Format Indicator Channel (PCFICH), Physical Downlink Control Channel (PDCCH)) are channels for providing control information needed by a UE to receive a PDCCH or for transmitting an ACKnowledged/Not ACKnowledged (ACK/NACK) message for a Hybrid Automatic Repeat reQuest (HARQ) operation in relation to an uplink data transmission.
In addition to the above signals, in LTE-A, muting may be configured to permit UEs within a corresponding cell to receive CSI-RSs sent by a different eNB without Interference. Muting can be applied to positions designated for CSI-RS, and a UE may receive a traffic signal while skipping a radio resource with configured muting. In the LTE-A system, muting may be referred to as a zero-power CSI-RS. Muting in itself is applied to the CSI-RS position without transmit power distribution.
In FIG. 2, the CSI-RS may be transmitted using some of the positions marked by A, B, C, D, E, F, G, H, I and J according to the number of antennas for CSI-RS transmission. Muting may also be applied to some of the positions A, B, C, D, E, F, G, H, I and J. In particular, the CSI-RS can be sent via 2, 4 or 8 REs according to the number of antenna ports for transmission. For two antenna ports, one half of a certain pattern is used for CSI-RS transmission; for four antenna ports, the whole of a certain pattern is used for CSI-RS transmission; and for eight antenna ports, two patterns are used for CSI-RS transmission. In addition, muting is always applied on a pattern basis. That Is, although muting may be applied to plural patterns, it cannot be applied to a part of one pattern unless the muting position overlaps the CSI-RS position. Muting may be applied to a part of one pattern only when the muting position overlaps the CSI-RS position.
In the case of CSI-RS transmission for two antenna ports, signals of the two antenna ports are sent respectively via two REs consecutive in the time domain and are distinguished from each other through orthogonal codes. In the case of CSI-RS transmission for four antenna ports, signals of two antenna ports are sent in the same manner as in the above case for two antenna ports and signals of the two remaining antenna ports are sent in the same manner via two additional REs. The above procedure may be applied to the case of CSI-RS transmission for eight antenna ports.
In a cellular system, a base station must send a reference signal to the mobile station for measurement of downlink channel states. In a 3GPP LTE-A system, the UE measures the status of the channel between an eNB and a UE by use of the CSI-RS transmitted by the eNB. The channel state is measured in consideration of several factors including downlink interference. Such downlink interference may include the interference caused by antennas of neighbor eNBs and thermal noise, and is important for determining the downlink channel condition. For example, in the case where an eNB with one transmit antenna sends a reference signal to a UE with one receive antenna, the UE must determine the energy per symbol that can be received in the downlink on the basis of the reference signal received from the eNB and the amount of interference that may be received simultaneously for the duration of receiving the corresponding symbol and determine the Energy per symbol to Interference density ratio (Es/Io). The determined ratio Es/Io is converted into a data rate or corresponding value, which is then reported to the eNB as a Channel Quality Indicator (CQI). Hence, the eNB may determine the data rate for downlink transmission to the UE.
In the LTE-A system, the UE feeds back information on downlink channel states to the eNB, so that the eNB may utilize the feedback information for downlink scheduling. That is, the eNB measures a downlink reference signal sent by the eNB and feeds back information extracted from the measurement to the eNB according to a rule specified in the LTE/LTE-A standard. In LTE/LTE-A, three pieces of information are fed back by the UE in general as described below.
Rank Indicator (RI) is a number of spatial layers available to the UE in the current channel condition.
Precoder Matrix Indicator (PMI) is an index to the precoding matrix preferred by the UE in the current channel condition.
Channel Quality Indicator (CQI) is a maximum data rate available to the UE in the current channel condition. The CQI may be replaced with a parameter similar to the maximum data rate, such as Signal to Interference plus Noise Ratio (SINR), maximum error correction coding rate associated with a modulation scheme, or data rate per frequency.
The RI, PMI and CQI are associated with each other in meaning. For example, the precoding matrixes supported in LTE/LTE-A are defined differently for different ranks. Hence, the interpretation of the same PMI value when the RI is set to 1 is different from that when the RI is set to 2. In addition, when determining the CQI, the UE assumes that the PMI and RI reported to the eNB are applied at the eNB. For example, if the UE has reported RI_X, PMI_Y and CQI_Z to the eNB, this means that the UE is capable of receiving data at a data rate corresponding to CQI_Z on the assumption of rank RI_X and precoding PMI_Y. In this way, the UE may assume a transmission scheme to be used by the eNB and calculate the CQI so that optimal performance can be obtained when actual transmission is performed using the assumed transmission scheme.
In LTE/LTE-A, periodic feedback of the UE may be configured as one of the following four feedback modes (or reporting modes) according to the information to be included therein:
reporting mode 1-0 reports RI and wideband CQI (wCQI);
reporting mode 1-1 reports RI, wCQI, and PMI;
reporting mode 2-0 reports RI, wCQI, and subband CQI (sCQI); and
reporting mode 2-1 reporting RI, wCQI, sCQI, and PMI.
The feedback timing in each feedback mode is determined based on the values transmitted through higher layer signaling such as Npd, NOFFSET,CQI, MRI and NOFFSETRI. In feedback mode 1-0, the wCQI transmission period is Npd, and the feedback timing is determined based on the subframe offset value of NOFFSET,CQI. The RI transmission period is Npd·MRI, and the offset is NOFFSET,CQI+NOFFSETRI.
FIG. 3 illustrates feedback timing of RI and wCQI when Npd=2, MRI=2, NOFFSET,CQI=1 and NOFFSETRI=−1. In FIG. 3, each timing (0-20) is indicated by a subframe Index.
In this case, feedback mode 1-1 has the same timing as feedback mode 1-0 with the exception that the PMI is transmitted together with the wCQI at the wCQI transmission timing.
In feedback mode 2-0, the sCQI feedback period is Npd and the offset is NOFFSET,CQI. The wCQI feedback period is H·Npd and the offset is NOFFSET,CQI as in the case of the sCQI offset. Here, H=J·K+1 where K is a value transmitted via higher layer signaling and J is a value determined based on the system bandwidth. For example, J is set to 3 in the 10 MHz system. This means that the wCQI is transmitted once at every H sCQI transmissions as a replacement of the sCQI. The RI period is MRI·H·Npd and the offset is NOFFSET,CQI+NOFFSET,RI.
FIG. 4 illustrates feedback timing of the RI, sCQI and wCQI when Npd=2, MRI=2, J=3 (10 MHz), K=1, NOFFSET,CQI=1, and NOFFSET,RI=−1. Feedback mode 2-1 has the same timing as feedback mode 2-0 with the exception that the PMI is transmitted together with the wCQI at the wCQI transmission timing.
Unlike the above feedback timing applied to the case of up to 4 CSI-RS antenna ports, for a UE associated with 8 CSI-RS antenna ports, two PMIs must be fed back. In the case of 8 CSI-RS antenna ports, feedback mode 1-1 is divided into two submodes. In a first sub-mode, the first PMI is transmitted together with the RI and the second PMI is transmitted together with the wCQI. In this case, the feedback period for the wCQI and second PMI is set to Np and the offset is set to NOFFSET,CQI, and the feedback period for the RI and first PMI is set to MRI·Npd and the offset is set to NOFFSET,CQI+NOFFSET,RI. In this case, if the precoding matrix corresponding to the first PMI is W1 and the precoding matrix corresponding to the second PMI is W2, the UE and the eNB share the information indicating that the precoding matrix preferred by the UE is determined as W1W2.
For 8 CSI-RS antenna ports, feedback mode 2-1 employs a Precoding Type Indicator (PTI) as new information. The PTI is transmitted together with the RI at a period of MRI·H·Npd with an offset of NOFFSET,CQI+NOFFSET,RI.
Specifically, for PTI=0, all of the first PMI, the second PMI and the wCQI are transmitted. In this case, the wCQI and the second PMI are sent together at the same time at a period of Npd with an offset of NOFFSET,CQI. The first PMI is transmitted at a period of H′·Npd with an offset of NOFFSET,CQI. In this case, H′ is transmitted via higher layer signaling.
For PTI=1, the PTI and RI are transmitted together. In this case, the wCQI and the second PMI are transmitted together, and the sCQI is transmitted at a separate time. In this case, the first PMI is not transmitted. The PTI and RI are transmitted at the same period with the same offset as the case of PTI=0. The sCQI is transmitted at a period of Npd with an offset of NOFFSET,CQI. The wCQI and the second PMI are transmitted at a period of H·Npd with an offset of NOFFSET,CQI, and H is set to the same value as the case of 4 CSI-RS antenna ports.
FIGS. 5 and 6 illustrate feedback timings respectively for PTI=0 and PTI=1 when Npd=2, MRI=2, J=3 (10 MHz), K=1, H′=3, NOFFSET,CQI=1 and NOFFSET,RI=−1.
In general, for FD-MIMO employing a large number of transmit antennas, the number of CSI-RS transmissions should increase in proportion to the number of transmit antennas. For example, in LTE/LTE-A, when 8 transmit antennas are used, the eNB must transmit CSI-RSs corresponding to eight ports to the UE for downlink channel state measurement. In this case, to transmit CSI-RSs corresponding to eight ports, a radio resource having 8 REs in one RB must be allocated for CSI-RS transmission as indicated by the positions marked by A and B in FIG. 2. When the CSI-RS transmission scheme of LTE/LTE-A is applied to FD-MIMO, a radio resource must be allocated in proportion to the number of transmit antennas for CSI-RS transmission. That is, an eNB having 64 transmit antennas must transmit CSI-RSs by use of 64 REs. Such a CSI-RS transmission scheme generating feedback information for each CSI-RS consumes excessive feedback resources. Hence, there is a need for a scheme that uses fewer feedback resources.